The lineup was decided based on the results of national, New Hampshire and Iowa polling. To qualify for the prime-time debate, a candidate had to place in the top six in an average of recent national polls, or in the top five in an average of recent Iowa or New Hampshire polls.

The debate comes as front-runner Trump faces a rising challenge from Cruz, particularly in the caucus state of Iowa where the two are effectively tied for the lead.

The changing dynamic has fueled new tensions in the race, with Trump now openly questioning whether Cruz’ Canadian birth complicates his eligibility to run.